1. Field of the Invention
The present invention relates to a method for creating a micromechanical membrane structure in a silicon substrate in which a cavity is created in the substrate's back side in a DRIE (deep reactive ion etching) process, and the present invention further relates to MEMS components manufactured using the method according to the present invention.
2. Description of the Related Art
The DRIB process is described in published German Patent document DE 42 41 045 C1 as a method for anisotropic etching of silicon substrates. The goal of this method is to perform the etching only at a right angle to the substrate surface if possible. With appropriate masking of the substrate surface, structures having a relatively large aspect ratio may be created in this way.
The DRIE process is a time-controlled two-step process in which an etching step alternates with a passivation step. These two process steps are regulated independently of one another. The etching step includes an isotropic chemical etching process, and an anisotropic physical material removal, which attacks essentially at a right angle to the substrate surface, superimposed on the chemical etching process. The DRIE process provides for stopping the etching step after a short period of time to create a passivation layer on the masked and structured substrate surface. The vertical side walls of the etching recesses created in advance are also passivated here in particular. Therefore the side walls are protected against the chemical etching attack of the next etching step while the passivation layer on the horizontal surfaces of the etching recesses is removed again by the physical material removal, so that a chemical etching attack may take place here. The sequence of etching step and passivation step is repeated as often as needed until the intended structure depth is reached.
It is known that cavities may be created in the back side of a silicon substrate with the aid of the DRIE process, thereby exposing membranes in the substrate's front side. However, this procedure has proven to be problematical in two regards. Since the DRIE process is a time-controlled process, the variance in the achieved etching depth and thus also the variance in the membrane thickness are relatively great with the given process parameters. Furthermore, the bottom of the cavity, i.e., the membrane's back side, is relatively uneven due to the process. Both of the aforementioned aspects have undefined effects on the mechanical properties of the membrane created in this way. However, further processing of such a membrane has also proven to be problematical. For example, the process management with subsequent structuring depends to a significant extent on the membrane thickness. Furthermore, the use of lithographic methods for definition of the membrane structure requires a preferably planar membrane surface or membrane's back side when the structuring is to take place starting from the substrate's back side.
In practice, membranes are often created in the front side of a silicon substrate in an anodic KOH back-side etching process. For this purpose, an n+-doped area extending over the entire membrane surface area and functioning as an etch stop limit for the KOH etching process is exposed in the top side of the substrate. Although membranes having a well-defined thickness and a very planar back side may be manufactured in this way, the rear opening in the cavern thus created beneath the membrane must be much larger than the membrane surface area because openings in the form of truncated pyramids are formed during KOH etching. This limits the miniaturization options of MEMS components manufactured accordingly.